What are 'Icing Conditions'?
 

Ok, so before anyone replies "Conditions where icing exists", I want to know what is meant by the term in the flight manual 'operation in known icing condition is prohibited'. My company, as a general rule use 5 degrees or less in visible moisture but i have heard others say you are ok to 0 degrees in moisture. Can anyone, using their encyclopedic knowledge and experience illuminate me on what temperature you can LEGALLY fly in visible moisture to remain within the parameters detailed in the AFM????

Obviously, water can't freeze until it meets a temperature of 0 degrees C.

Can this occur in the engine intakes, or plenums, for example? If so, you might encounter an engine icing situation although the airframe remains ice free.

Water can exist in a liquid state below that temperature, if there is nothing for droplets to freeze onto or around (supercooling). If the airframe, or part thereof, is below zero and it meets supercooled water droplets - you then almost certainly have icing conditions.

One other consideration - how accurate is the OAT guage on a particular aircraft?

In my outfit, if any build up is noticed (the wiper blades are an excellent indicator) THAT is icing conditions. However, as always, the Flight Manual is the paramount authority.

According to some, snow is OK :} :rolleyes:

Cheers

Whirls

I would imagine it is when the met office report that they exist. And bear in mind that the FAA regard "forecast icing conditions" as "known icing conditions".

You could also infer it from the flight manual from when the anti-icing needs to be on - e.g. +4.4C for the 206, but Robinson have also issued warnings for +30 in humid conditions, so my own inclination to take the one the jury would take notice of!

Phil

I'm not sure that it is against the law to fly in visible moisture (icing) conditions or outside the parameters detailed in the AFM.

I am sure it would be unwise though.
:hmm:

Like Phil says, the met reports are a great place to start looking for icing conditions, be it forecast icing or pireps showing icing encountered by others inflight.

Icing is an odd thing though and can be found in a range of conditions. I've flown IMC at -2C and never picked up any ice at all. On another day I flew thru a small localized rain shower at +5C and to my surprise watched the water freeze on contact with the window. :eek:

Be careful. Icing is defined in Appendix C of Part 27 and 29, but it only covers water droplet sizes up to 50 microns - that's 0.5mm (or the thickness of a mechanical pencil lead). In that condition, the droplets are more or less suspended in mid-air.
Anything larger than 0.5mm is considered to be freezing precipitation - and no aircraft (fixed or rotary wing) is cleared for flight in those conditions. So if it's not snow, and you encounter something falling from the sky and freezing, stay on the ground.

Depends on the jurisdiction. Like Paco says, different authorities have different definitions. Some look only at an OAT gauge, some look for ice on sponsons or wipers, some look at the torque gauge, some look at a weather forecast. In some places regardless of what is forecast, if some brave ice-dummy soul ventures up and reports back that he isn't picking up ice then to the rest of the pilots it is no longer "Known Icing", in fact it is now "Known Not To Be Icing".

Pilots in places like Norway or the Canadian North that wouldn't turn a blade from September to April following the rules of more temperate locales, have built up a vast body of knowledge based on experience. Such as Whirl's comment on snow. This is passed down from senior pilot to junior pilot. As it should be.

The FAA has recently produced a legal interpertation defining known ICE for flights in the USA. As most guidance from the FAA it is not very usfull, if you get in trouble on a flight in iceing conditions you get hung out to dry. If the flight is completed ok you are ok, go figure.

The FAA also defines known ice in the Aeronautical Information Manual as ice observed accumulating on the aircraft.

The link for the interpertaion is below.






Regulations

Isn't that 500 microns? Or 0.05 mm?

Shytorque, I guess you do know already that ice can form at *ambient* temperatures above zero where there is a cooling effect from expansion - the obvious example being carb icing. But would such an effect occur on the airframe? Any low pressure area could be a candidate, such as behind slots in flaps, behind objects projecting into the airstream etc.

Yes, I am very much aware of the individualities of airframes. Follow Whirls' link and look at my argument on that discussion, which was in a nutshell that generalities are dangerous things to believe. In that instance one contributor was quite adamant that snow could not stick to an airframe - I disagreed. Icing trials, particularly on helicopters, need to be very carefully carried out for a meaningful icing clearance to be issued. Subsequent experience in a variety of conditions may result in a clearance being further restricted or eased.

Don't quote me, but I think there is a UK definition of Icing Conditions as "flight in visible moisture with an OAT of 0C or less, with a visibility of 1km or less".

JHR - you are right, that letter from the FAA isn't overly helpful. On one hand it says that known icing conditions only exist when you see ice accumulating on the airframe but on the other they say a pilot who flys into an area where icing would probably be encountered (based on met forecasts and reports) is OK if they don't encounter ice but might be prosecuted if they do!

If potential icing conditions exist ie less than 1000m vis with OAT below zero - then as long as you have what the FAA call 'an icing exit strategy' you are pretty much acting within the letter and the spirit of the RFM.

The problem with helicopters is that once you start to accrete ice you often don't have the power/performance to climb and only a limited ceiling anyway so you are forced into descent into clear/non icing air or a turn back to a known clear/non icing environment.

So if your enroute met looks icy all the way to the deck it might be time to say no.

err... accrue, shurely?

fkelly

ice 'accretes' to an aircraft ie attaches itself to the airframe. your bank account 'accrues' interest or more likely if like most of us on here...your scarily increasing overdraft accrues charges!!! :E

biz

Icing Conditions
 

Shytorque mentioned that "you might encounter an engine icing situation although the airframe remains ice free" ... the following definitions appear in one of my company's limitations documents:

"Rotor and airframe icing conditions are defined as cloud or fog in the temperature range 0ºC to –30ºC when the visibility is less than 1,000 metres" (where –30ºC is the bottom end of the aircraft environmental envelope). However ... "Engine icing conditions are defined as cloud or fog with visibility less than 1,000 metres, or rain, all in the temperature range of +10ºC to –30ºC". (The increased upper limit (+10ºC) recognises the cooling effect of the engine intakes ...)

The same document also includes the following comment: "Due account must be taken of the limit of accuracy of the OAT indication, which is ±1ºC".

CY

"Flight in icing conditions prohibited", so says the Flight Manual.
I read this, and luckily so does my company as "conditions conducive to icing" i.e. 0 degrees, visible moisture, viz below 1000m etc. If such conditions are forcast on your route you should avoid it. By penitrating the area you are in breach of the limitation section in the flight manual.

The word "conditions".

As a verb: To make dependent on a condition or conditions.
As a noun: a circumstance indispensable to some result; prerequisite; that on which something else is contingent.

Both come up with the same answer. a number of prerequisites occuring together to give in our case Icing.

There is a subtle difference between flight in icing conditions and flight in icing.
The first, your going to get it, the second, you've got it.

Check - yes but Eurocopter beans' wording at the start of this thread was 'known icing conditions' which is more specific then 'icing conditions'.

You interpret your RFM to read 'possible icing conditions' which I have no problem with but, just because you are in cloud below zero degrees, it doesn't mean that you will pick up ice because more often than not you don't.

Crab, your right, I never re-read the original post. However I still stand by my post.

We have three conditions available, no icing, icing conditions, and icing. The only way of being informed of known icing conditions are a) entering into the condition of flight in icing conditions where you may or may not pick up ice, or from reports from other aircraft that have done just that and picked up ice. If the met report reports icing then what's the question?

The wise will carefully read met reports and if icing conditions exist will ensure that if he/she decides to procede then they have to ensure enough space below to leave the icing and continue or return as the case may be.

This icing thing comes round every winter and most of us seem to handle it well, survival has something to do with it!

B206 Icing: A Critical Question!
 

Hi guys,
I have been looking at the Bell 206BIII RFM and it says to turn on pitot heat and engine de-icing at OAT below 4.4 celsuis and visible moisture. Now, according to the manual flight is prohibited into 'known icing conditions'.

The questions that arise are:
1. What is visible moisture?
We have eyes for that, obviously and we know that mist, clouds, rain, snow and fog all are forms of visible moisture. There can also be an estimation of humid conditions by comparing the temp/dewpoint spread. I am looking for a specific definition, if any; as I have heard something related to visibility!
(I do not agree with having a definition when we have the word 'VISIBLE' in the manual)

2. What are known icing conditions?
Central Europe has icing conditions all winter. The zero-degree isotherm is always at SFC and icing conditions are always present. What would it be? PIREPs? AIREPs?
(To me its an area suspect-able to icing, like clouds, freezing rain or high altitude freezing areas)


Would really love to hear what other people have to say!
Thanks,
XR

Visible moisture - what it says - mist, drizzle, hail, etc.

To the FAA, forecast icing conditions = known icing conditions. In court, in Europe, I suspect that known icing conditions would include where it could be reasonably expected that you would get icing, even if there wasn't a forecast to hand.

phil

Known icing conditions are just that. If you fly and collect ice, that is known icing conditions. Some days the forecast can be incorrect, or just not accurate enough. A half a degree C can make the difference between collecting ice or not. You could change your track by a half a mile and be in (or out) of icing.

If the forecast indicates icing may take place, that should be used as a (strongly advisable) guide.

BTW, the engine has anti-icing protection, not de-icing. Anti-icing prevents ice formation. By definition, de-icing insinuates that you have already flown in icing and collected some ice.

To add an importnt point. Anything larger than 0.5mm droplet size is freezing precipitation - and definitely must be avoided, even if you have an icing clearance.
A good rule of thumb for this is that if the moisture in the air is descending (as opposed to drifting around), then the droplet size is larger than 0.5mm.
Freezing mist is bad enough, freezing precipitation is much, much worse.

Shawn, just to pick up on your point about freezing precipitation, I quite agree that flying in this stuff is a seriously bad idea, but I query whether its correct to say that no heli is cleared to fly in that stuff? There is nothing in the Eurocopter flight manuals about it (though flight in limited icing conditions is allowed with certain caveats). There used to be something in our Operations Manual prohibiting it, but its been lost along the way somewhere.

I have been asked about whether we can fly in freezing precipitation. Of course my answer is no, but I am at a loss to find written justification as to why!

So my question is, where does it specifically say that you can't fly in freezing precipitation? Or are you just taking the line that since it doesn't say you can, you can't.

I would also take the view that flight in snow or hail or ice crystal cloud is not icing conditions, provided the temperature is well below zero (if near zero, snow/hail can melt on impact and then re-freeze). In other words, if it doesn't stick it isn't icing conditions!

HC

As the question is "LEGALLY ice conditions" I think that we shall consider the ufficials forecast informations. IMHO

WX.............: SCT SN/SNRA, LATE NIL CENTRAL PARTS,
RISK LCA FG/FZFG
VIS............: +10KM, LCA 2-7KM IN WX,LCA 0100-1000 M IN FG/FZFG
CLD............: FEW-SCT 1000-1500FT, SCT-BKN 1500-5000FT,
LCA BKN/VV 0800-1500FT IN WX,
LCA VV002-009 IN FG/FZFG
0-ISOTHERM.....: SFC-1000FT, BECMG SFC-2000FT
ICE............: FBL/MOD, LOC MOD, LATE BECMG FBL/NIL

This is from today's IGA prognosis at the western part of Norway.
It is forecast icing conditions, but we are still flying VFR ops with 350's. The crew reported light icing in snowshowers on the last trip, but went out again.
One has to fly in visible moisture to get ice, unless we talk about carb.ice. That's another story.
But if you push it, and hover in light fog in minus-degrees, you may get icing very quick! There have been a few accidents because of that.

One R-22 was doing run-up in light freezing fog, and when they were ready to depart, they couldn't get the heli in the air. They shut down again, and revealed a lot of ice on the blades.... What they were supposed to do, flying in freezing fog, is another question....:mad:

A 350 flew in -20 c in Northern Norway, blue skies. They were to drop of some workers in the bush, and there was light fog at the landingsite. They landed, let of the passengers, and took off again. They didn't get enough lift, and crashed a couple of hundred meters away. They got icing during the short ground stop.

We are very aware of the OAT during the winter when there's visible moisture. As some has pointed out, ice on the wipers is a good indicator.:)

I agree with those who say airframe icing conditions are generalised as below 0 degrees Celsius and in less than 1Km viz (i.e. fog).

I have heard that the term 'visible moisture' by definition is moisture causing a visibility of less than 1Km, although i would be hesitant to fly in mist (1-4km) at less than 0 degrees, which incidentally i would say is quite visible :}

with regards to engine/carb icing, the temp drop across the carb can (i've heard) be as much as 30 degrees so in humid conditions one must be very careful not to get caught out flying in +30C....

also there is obviously the possibility of freezing rain under the passage of a warm front, if you're in the cold sector, and rain from the warmer sector above cools to below zero (supercooled) and sticks upon impact, or your airframe is very cold and fly's into the rain... leading to particularly dangerous clear ice. :eek:

Aucky

For what it's worth the CAA Supplement to the R22 POH says:

Icing conditions must be assumed to exist when in cloud, or when the visibility at takeoff is reduced to 1000 metres or less in visible moisture, with a true air temperature of 0°C or less. Icing conditions must also be assumed to exist whenever there are any visible signs of ice or slush build up on any part of the helicopter.

It goes on to state: Flight in freezing rain or freezing drizzle regardless of visibility is prohibited.

That'll do for me

Helicomparator:
Like many things, it's not well defined. I know the Canadian FM for the Super Puma was amended to say No flight in freezing Precipitation, not sure about the others.
The logic is as follows:
The clearance for icing is based on testing to the requirements of one of the Appendices to Part 27 and Part 29 (can't remember which letter it is). This is the same wording and requirements as far as droplet size is concerned as the FW world uses. The maximum droplet size used for certification is 0.5mm, and anything larger is considered to be freezing precip. No one has a clearance to fly in freezing precipitation.
Hope that helps!

Because, if you haven't realised from some of the posts, of the rapid buildup of ice on the airframe/rotors in freezing rain/drizzle. This leads to an increase in weight and power(to cope with the extra weight) and loss of a nice aerodynamic profile(and hence lift) on the rotor blades.

It's bad stuff: don't go there.

AOPA Online: In-Flight Emergencies: Avoiding ice fright

Aircraft icing (xhtml w3c 11/09)

More confusion.

I have it written down that the CAA in their publications believe Icing conditions are when the temperature is below +3°Celsius and visible moisture below 1500m sadly I can't put my finger onto the CAA document at the moment. Maybe after the weekend.

On the last page of the AS365 FLM it mentioned 4°Celsius if I remember correctly.

Droplet Size and Implications
 

Shawn, believe you are correct re the 50 micron droplet size being the max for icing certification.

HC, the problem with freezing rain is what has been called "runback", wherein the precipitation runs back beyond the aft limit of the de-icing system chordwise coverage and freezes there*. Creates both lift and pitching moment problems.
* In fact preventing runback from occurring within the normal icing conditions envelope is a major controlling factor in the de-icing element timing cycle development.

Thanks,
John Dixson

Some of you are probably aware of this but the standard weather service methodology and terminology used to characterize and classify the icing environment was developed from in-flight icing tests conducted on a Douglas DC4 and DC6 type aircraft. Thus labels such as trace icing, light icing, moderate icing and heavy icing related to the rate of ice accretion on a probe on a DC6, which does little to ascertain or predict the rate of ice accretion on a rotor system.
As an example, light icing is defined as an accumulation of one half inch of ice on a probe after 40 miles of flight. The rate of accretion is sufficient to create a hazard if flight is prolonged in these conditions, but insufficient to require a diversion.
The prior definition may well fit a large aircraft however there is no assurance that the rotating surfaces of a helicopter will accumulate only one half inch of ice over the same 40 miles, indeed there are many more factors in play with ice accretion to a rotor surface as opposed to a single non rotating airfoil that make it a much larger problem in much less time. Further while one half inch of ice on the wing of an airplane may correctly and appropriately be called “light icing” there is every reason to believe from testing that one half inch of ice on the leading edge of most helicopter rotor systems could result in tragic consequences if autorotation became necessary.

Mightygem, JohnDixon et al

I guess I didn't make myself clear. In no way was I suggesting that flying in freezing precipitation was anything other than a very bad idea, and I am well aware of the seriously nasty effects that could rapidly arise if one foolishly did so.

My question only related to where in the legislation etc it is written down as being prohibited, so as to provide an answer to those that have asked me "but where does it say that?" when I have told them flying in FP is verboten.

HC

HC - my answer would be that you can't because there is no specific clearance to say you can. Since every pilot from day one is taught the dangers of freezing rain and drizzle it seems reasonable to argue that therefore being able to fly in it would require some special equipment and/or clearance. You can anti-ice your engines and you can de-ice your MR and TR blades (with the right fit) but to my knowledge there is no method of de-icing either the airframe or the pitch change mechanisms of the rotors, both of which will be badly affected by freezing precipitation.

I have only encountered freezing rain once and then only briefly but it was more than enough to convince me that all the warnings are valid.

It is always interesting to see which pilots go for 'I can do it because it doesn't say I can't' versus 'I can't do it because it doesn't say I can' when quoting rules and regulations.

HC, dug up some legislation wording used here in the great frozen north were nobody stops flying.

Transport Canada's latest position on freezing rain or drizzle is that it is equivalent to "severe icing". Quote from ASL 4/2009 "Takeoff into known freezing drizzle and/or light freezing rain is outside of the flight envelope for which any airplane currently operating today is certificated. ...".

Interestingly (or practically for operations in Canada), they differentiate between "forecast" icing, and "known" icing. In fact currently the regs 605.30 says
"no person shall conduct a takeoff or continue a flight where icing conditions are reported to exist or are forecast to be encountered along the route of flight unless
a) the pilot-in-command determines that the aircraft is adequately equipped to operate in icing conditions in accordance with the standards of airworthiness under which the type certificate for that aircraft was issued; or
b) current weather reports or pilot reports indicate that icing conditions no longer exist"

There is a proposal to change (b) further to permit more flexibility in operating in reported icing conditions:"b) current weather reports, pilot reports, or briefing information relied upon by the pilot-in-command indicate that the forecast icing conditions that otherwise prohibit flight will not be encountered during the flight because of changed weather conditions since the forecast." In other words, even if icing is forecast, if the commander has reason to believe his aircraft won't ice up along his route he can still fly.

So there you go for all you ice-dummy aircraft commanders. "heavy lies the crown..."

Look in the RFM. ;)

Water can remain liquid in 'icing conditions' i.e. below 0 deg C or 32 deg F but impact with an object (like a main rotor blade or airframe) can cause freezing into a solid. However, the "freezing" does not occur all-at-once. In fact, there is a substantial lag.

I expect you all know this already, it forms part of the knowledge associated with ice detection systems like the Leigh Ice Detector.

Icing Regs and Arguments
 

HC, of course the RFM is the document as called out in 29.1581, but I suspect that you were getting at some of the definition arguments.

See the following for examples of the associated legal tangles:

The Naked Truth About Known Icing Conditions.

What fun.

Thanks,
John Dixson